1. Field of the Invention
The present invention relates to a grinding machine and a grinding method for grinding the tooth flanks of a gear-like workpiece as a stock material with a gear-shaped grinding stone engageable with the particular tooth flanks in the process of manufacturing a gear-like product having internal teeth or external teeth.
2. Description of the Related Art
A method for grinding the tooth flanks of the internal teeth, such as a wire cut method or an electric discharge machining, is well known in the process of manufacturing a product having internal teeth such as an internal gear, a ring gear or an internal spline. The problem of the prior art, however, is that a long machining time is required and therefore volume production is difficult with small equipment. Another problem is a high price due to a high cost caused by an increased size of the machining equipment for volume production.
In the grinding machine or the grinding method for grinding the tooth flanks of a gear-like object to be machined (workpiece), having internal or external teeth, with a grinding stone engageable with the particular gear flanks, a contact pressure of a predetermined magnitude is required to be generated in the machining section in which the tooth flanks of the gear-shaped grinding stone and the tooth flanks of the gear-like workpiece to be ground are in contact with each other. A conventional grinding method is known as a technique meeting this requirement, in which a gear-shaped grinding stone and a gear-like workpiece in mesh with each other are rotated simultaneously in complete synchronism with each other. However, the mechanism for synchronously rotating the grinding stone and the workpiece requires a high accuracy, and therefore the machining equipment cost is very high. Thus, this method is not economical from the viewpoint of the machining cost.
In a comparatively simple conventional method described in Japanese Unexamined Patent Publication (Kokai) No. 34-1650, on the other hand, a gear-shaped grinding stone is rotated thereby to cause the free running of a gear-like workpiece in mesh with the grinding stone, while the driven workpiece is braked thereby to exert a resistance (rotational resistance) of a predetermined magnitude on the rotation of the workpiece. The contact pressure required for grinding is thus generated in the machining section, thereby grinding the tooth flanks of the workpiece.
The prior art using the free-running method described in the patent publication cited above, however, cannot successfully meet the unavoidable deterioration with age including the reduced sharpness of the grinding stone and the reduced rotational resistance to the gear-like workpiece caused by the wear of the brake. Also, the prior art, which never intends to control the magnitude of the rotational resistance with high accuracy, harbors a problem that the tooth flanks of the workpiece cannot always be finished with high accuracy.
An object of the present invention is to cope with the problem of the prior art described above and to provide an improved internal tooth grinding machine which can grind the internal teeth at high speed with small machining equipment while at the same time making possible volume production with high accuracy at low cost.
Another object of the invention is to provide a grinding machine constituting a simple, inexpensive machining means employing the free-running method as in the prior art described above, wherein a sufficiently high machining accuracy can be maintained in keeping with the deterioration with age including the reduced sharpness of the grinding stone and the reduced rotational resistance due to the brake wear, thus making it always possible to grind a gear-like workpiece with high accuracy on the one hand and a novel method of grinding a gear-like workpiece on the grinding machine on the other hand.
In order to achieve the objects described above, according to one aspect of this invention, there is provided an internal tooth grinding machine for grinding the tooth flanks of the internal teeth of a workpiece having a circular outer periphery and a gear-shaped inner periphery in such a manner that the workpiece is caused to engage with a grinding stone having external teeth formed on the outer periphery thereof, and one of the grinding stone and the workpiece is rotated with the other run free, while at the same time reciprocating one of the grinding stone and the workpiece relatively to the other in an axial direction, the internal tooth grinding machine comprising:
at least two rollers located on the outer periphery of the workpiece for holding the workpiece at a machining position by holding the workpiece in collaboration with the grinding stone located on the inner periphery of the workpiece;
a plurality of bearings for rotatably supporting the rollers, respectively;
a block for supporting the bearings; and
a roller support unit for pressing the workpiece against the grinding stone through the blocks and the rollers.
The internal tooth grinding machine according to the invention basically operates in such a manner that one of the gear-shaped grinding stone and the workpiece having internal teeth is rotated while the other, in mesh, is run free thereby to grind the tooth flanks of the workpiece. In the case where only the contact points of the tooth flanks, formed by free running, are ground, the force is concentrated only on the narrow contact points between the external tooth flanks of the grinding stone and the internal tooth flanks of the workpiece. In that case, only the slight slip between the tooth flanks of the grinding stone and the workpiece contributes to the grinding work and, therefore, high-speed, high-accuracy grinding would be impossible. In the internal tooth grinding machine according to the invention, on the other hand, not only one of the grinding stone and the workpiece is run free but also both are reciprocated relatively to each other in the axial direction at the same time, and therefore all the parts of the gear profile including the tips and bottoms of the workpiece can be ground at high speed with high accuracy. The effect of the reciprocal motion is strengthened by the fact that a force is exerted on the contact surface between the grinding stone and the workpiece due to the free running. In other words, the free running and the reciprocal motion have a multiplier effect.
In the internal tooth grinding machine according to the invention, a workpiece with a circular outer periphery is supported rotatably by at least two rollers. Therefore, the outer periphery of the workpiece and the parts of the grinding machine supporting the workpiece are prevented from wearing. As a result, the machining accuracy and the durability of the grinding machine are further improved.
An alternative method for running the workpiece or the grinding stone freely while at the same time reciprocating them relatively to each other along the rotational axis consists in rotating the grinding stone with a grinding stone support unit while at the same time causing the same grinding support unit to drive the grinding stone to reciprocate along the axial direction. Still another method is to drive the rollers in the rotational direction by a rotation drive unit while running the workpiece and the grinding stone freely. Preferably, however, the grinding stone rotatably supported by the grinding stone support unit is preferably reciprocated in the axial direction.
The internal tooth grinding machine according to the invention, which usually employs at least two rollers, desirably comprises three or more rollers for stabilizing the supported condition of the workpiece leading to a further improved the workpiece machining accuracy and the durability of the grinding machine. Further, in the case where the machine comprises three or more rollers, at least one block supporting a roller is separated from the other blocks supporting rollers, so that the former is movable relative to the latter in the radial direction of the rollers. In this way, the workpiece can be conveniently mounted at the machining position between the rollers or recovered from the machining position after complete machining process by changing the intervals between the rollers.
In any case, by forming an annular groove along the outer peripheral surface, i.e. along the peripheral direction of the rollers for engaging the workpiece, the workpiece would not come off from the rollers thereby making it possible to perform the grinding work both positively and accurately.
According to the invention, there is also provided a gear grinding machine having the following configuration as other means for solving the problems described above.
The grinding machine according to the invention comprises:
means for detecting the magnitude of the grinding resistance exerted along the axial direction of a gear-shaped grinding stone in such a manner as to interfere with the reciprocal motion of the gear-shaped grinding stone along the rotational axis; and/or means for detecting the magnitude of the rotational resistance exerted in the direction opposite to the rotational direction of the gear-shaped grinding stone in such a manner as to interfere with the rotational motion of the gear-shaped grinding stone;
means for applying the rotational resistance to one of the gear-shaped grinding stone and the gear-like workpiece or a gear-shaped grinding stone shaping member which is run free in such a manner as to interfering with the rotation of that one of the gear-shaped grinding stone and the gear-like workpiece or the gear-shaped grinding stone shaping member which is run free, respectively; and
means for controlling the rotational resistance application means;
wherein, when the grinding force of the gear-shaped grinding stone decreases to such an extent due to the secular wear of the gear-shaped grinding stone or the rotational resistance application means that the resulting increased grinding resistance is detected by the grinding resistance detection means or the resulting decrease in the rotational resistance is detected by the rotational resistance detection means, then the rotational resistance control means controls the rotational resistance application means thereby to increase the rotational resistance against the gear-like workpiece or the gear-shaped grinding stone shaping member.
As a result, the gear-like workpiece or the gear-shaped grinding stone shaping member is prevented from being relieved in the rotational direction, and the contact pressure is increased which is exerted on the machining section or the shaping section between the tooth flanks of the gear-shaped grinding stone and the tooth flanks of the gear-like workpiece or the gear-shaped grinding stone shaping member in mesh with the gear-shaped grinding stone. In this way, it is possible to prevent a reduction in the grinding capacity or the shaping capacity which otherwise might be caused by the secular wear of the gear-shaped grinding stone or the rotational resistance application means. Thus, the gear grinding work or the shaping of the gear-shaped grinding stone, which is always stable and high in accuracy, is made possible free of secular variations.
More specifically, the rotational resistance application means includes a brake unit, the grinding resistance detection means can be a torque sensor for detecting the torque of the motor for reciprocating the gear-shaped grinding stone along the axial direction, and the rotational resistance detection means can be a torque sensor for directly detecting the torque of the motor for rotating the gear-shaped grinding stone. Also, the rotational resistance control means can include arithmetic means for calculating the average value of each of the signals detected by the grinding resistance detection means and the rotational resistance detection means, comparing the average value with a predetermined reference value and applying a control signal corresponding to the difference to the rotational resistance application means. More specifically, these torque sensors can be configured with a current sensor for detecting the value of the current flowing in the motor.
According to still another aspect of the invention for solving the problems described above, there is provided a gear grinding method for grinding the tooth flanks of a gear-like object to be machined or shaping a gear-shaped grinding stone in such a manner that the gear-shaped grinding stone is caused to engage the gear-like object or a gear-shaped grinding stone shaping member, and one of the gear-shaped grinding stone and the gear-like object or the gear-shaped grinding stone shaping member, as the case may be, is rotated with the other run free, while at the same time reciprocating one of the gear-shaped grinding stone and the gear-like object or the gear-shaped grindings stone shaping member relative to the other along the axial direction, the method comprising the steps of:
detecting the magnitude of the grinding resistance exerted in such a manner as to interfere with the reciprocal motion while at the same time detecting the magnitude of the rotational resistance exerted in the direction opposite to the rotational direction of the gear-shaped grinding stone and the gear-like object or the gear-shaped grinding stone shaping member in such a manner as to interfere with the rotational motion of the gear-shaped grinding stone and the gear-like object or the gear-shaped grinding stone shaping member, respectively;
applying the rotational resistance to that one of the gear-shaped grinding stone and the gear-like object and the gear-shaped grinding stone shaping member which is run free in such a manner as to interfere with the rotation of one of the gear-shaped grinding stone and the gear-like object and the gear-shaped grinding stone shaping member, respectively; and
changing the magnitude of the grinding resistance and the magnitude of the rotational resistance toward a corresponding reference value by controlling the rotational resistance application means upon detection of a change of at least one of the grinding resistance and the rotational resistance;
wherein only the grinding resistance or the rotational resistance may be detected to control the rotational resistance application means in accordance with the change of the detected value.
The gear-like workpiece making up an object to which the gear grinding method described above is applicable is not necessarily confined to the workpiece of internal gear type, but may include an external gear, a bevel gear or another gear-like workpiece. The feature of the gear grinding method according to the invention lies in that when the gear-shaped grinding stone is reciprocated for grinding, the magnitude of the grinding resistance exerted in axial direction in a manner to interfere with the reciprocating motion is detected or the magnitude of the rotational resistance exerted in the direction opposite to the rotational direction of the gear-shaped grinding stone in a manner to interfere with the rotational motion of the gear-shaped grinding stone is directly detected; the rotational resistance is applied by the rotational resistance application means in a manner to interfere with the rotation of that one of the gear-shaped grinding stone and the gear-like workpiece or the gear-shaped grinding stone shaping member which is run free; and upon detection of a change in the grinding resistance or the rotational resistance, the rotational resistance application means is controlled thereby to change the magnitude of the rotational resistance toward a reference value.
As a result, the gear-like workpiece or the gear-shaped grinding stone shaping member is prevented from being relieved in the rotational direction, thereby increasing the contact pressure exerted on the machining section or the shaping section between the gear-shaped grinding stone and the gear-like workpiece or the gear-shaped grinding stone shaping member, as the case may be. Thus, it is made possible to prevent the grinding or shaping capacity from being decreased. Thus, the tooth flanks of gears of various shapes can be always stably ground with high accuracy free of secular variations.